The invention relates to a method for handling a call coming to terminal equipment suitable for speech and data service particularly in a digital transmission system, the transmission system comprising at least one connection element which is a speech transmission connection when the call is in speech service mode and which is a data transmission connection rate-adapted in compliance with the CCITT recommendation V.110 or in a like manner and preferably attached to V.24 interfaces when the call is in data transmission mode.
As used herein, "GSM" refers to Global System for Mobile Communications, a European digital mobile radio system, and "PLMN" refers to Public Land-Mobile Network.
In a GSM PLMN mobile telephone network, for instance, the radio connection between the mobile telephone and the fixed network will in data transmission be a V.110 rate-adapted UDI-coded digital connection attached to V.24 interfaces, and in speech transmission a PCM-coded digital connection. ISDN and GSM PLMN subscriber signalling will include an In-Call Modification procedure, by which the speech connection can, if required, be changed to a V.110 rate-adapted data connection and vice versa, an unlimited number of times. In a system of this kind, the terminal equipment may comprise, for instance, a telephone handset connected to a network termination (mobile telephone) or integrated in a data terminal for telephonic conversations and a data terminal adapted for a V.110 rate-adapted full-duplex connection by a terminal adapter.
Such terminal equipment suitable for both data and speech service include the Telefax Group 3 terminal equipment and personal computer telecommunications software which is adapted by means of a terminal adapter of a mobile telephone for instance to V.110 rate-adapted GSM PLMN radio connection.
The terminal devices of the Telefax Group 3 heretofore have not comprised a V.24 interface, but they have always utilized a modem interface and a 2-wire line (modem connection through a telephone network, for instance). As the same transmission line has in that case been able to transmit both modem signalling and speech, the terminal devices have been able to perform the transition from data service to speech service, for instance, in compliance with the service management protocol of the OSI layers 4 to 7 (for example CCITT T.30 or T.4) used in each case, without any need for the intermediate transmission network to have knowledge of the change of service mode or to react to such a change by altering the configuration of the transmission connection in any way. The service to be utilized has been manually selected from the terminal equipment. An incoming call to the terminal equipment has been started directly from this selected service.
In the GSM PLM, the control of an incoming call to terminal equipment has been so arranged that a mobile telephone whereto a data terminal is also attached selects speech or data service for the call by one of the following means:
1) The system has one catalogue number for each mobile telephone, the GSM PLMN connection being set up on the basis of the Bearer Capability (BC) information conveyed by the SETUP message sent. The BC specifies which service the connection is set up for. PA0 2) The system has one catalogue number for each service of a mobile telephone, the GSMN PLMN connection being set up on the basis of the catalogue number of the service.
The first of the above-stated call control modes functions faultlessly if the GSM system is connected to the public switched telephone network (PSTN) wherein ISDN signalling ISUP (CCITT recommendations Q.721-Q.725) capable of transmitting a BC is employed. However, during the initial years GSM mobile telephone networks will have to be connected to the PSTN for instance by means of interexchange common channel signalling TUP (CCITT recommendations Q.761-Q.765) which is not at all capable of transmitting a BC, in which case the latter call control mode should be employed. In certain countries, however, the numbering plans for telephone networks may be so restricted that also in this case the GSMN PLMN can only be numbered by assigning one catalogue number to each mobile telephone. In such a case, the problem resides in how terminal equipment suitable for speech and data service can respond to an incoming call with the correct service.